Presently, system development has proceeded in systems where radio communication terminals freely perform handover between systems of a mobile communication (cellular) system which allows a lower data transmission rate yet covers a wider area and a wireless LAN system which allows a higher data transmission rate yet covers a smaller area. Particularly, in 3GPP (3rd Generation Partnership Project) planning communication standards for mobile communications, studies are underway to standardize a handover scheme capable of continuing voice communication between circuit-switched voice communication and VoIP voice communication (see, 3GPP draft TR (Technical Report) 23.806, TS (Technical Specification) 23.206). Further, the technique disclosed in Patent Document 1 is known as a handover scheme between circuit-switched voice communication and VoIP voice communication.
To perform handover between a circuit-switched network supporting circuit-switched voice communication and an IP network supporting VoIP voice communication while enjoying telephone service, as shown in FIG. 12 of Patent Document 1, the steps of establishing a call for the target domain and then disconnecting the call from the source domain are used. In Patent Document 1, terminals calling each other perform call control, and, as studied in 3GPP TR 23.806, by relaying a call by one server or a plurality of servers in a network and by converting call control messages and converting medias between circuit-switched voice media and VoIP voice media, it is possible that handover is performed regardless of the call connection schemes communicating parties support.    Patent Document 1: Japanese Patent Application Laid-Open No. 2004-297591